A path traversal flaw in the generate_image() function of Banana Slides 0.4.0 lets attackers craft markdown image references that cause the application to read arbitrary image-format files outside the intended uploads directory. The flaw arises from an incomplete prefix check using os.path.startswith() without a trailing separator, enabling directory confinement bypass. As a result, unauthenticated attackers can read any accessible file that resides in a sibling directory whose name shares the uploads folder prefix, potentially exposing sensitive data.

The vulnerability affects Anionex Banana Slides version 0.4.0. The patch is implemented in commit e8bc490, which is included in later releases.

The CVSS score of 8.7 classifies this exploit as high severity. No EPSS data is available, and the vulnerability is not listed in the CISA KEV catalog. Attackers only need to supply a crafted image reference within an unauthenticated user‑controlled page description; the backend then opens the target file with PIL Image.open(), making the attack straightforward and requiring no additional privileges.